1. Field of the Invention
The present invention relates to a process for improving the impact resistance of plastic substrates, such as those used as ophthalmic lenses, by applying to at least one surface of the plastic substrate a primer layer consisting of an aqueous thermosetting polyurethane dispersion which can be cured by air drying at ambient temperature before another layer is applied over it.
2. Description of Related Art
Clear, transparent plastic materials are widely used as substitutes for glass in many applications in view of their unique properties, such as lighter weight, ease of handling, and ease of formation of articles. Plastic materials are not without shortcomings, however, as most plastics are soft and scratch quite readily. Consequently, it has become customary to coat plastic materials with organic or siloxane coating compositions to provide an abrasion resistant "hard coat" before using them in certain applications. Preferably, such hard coats are clear, unpigmented coatings. Exemplary organic coatings include heat curable epoxy and polyurethane resins and ultraviolet light curable acrylic resins. Although such organic coatings tend to be less expensive than siloxane coatings, their abrasion resistance generally is not as good.
Many siloxane coating compositions are known which, when applied to a substrate and cured, provide a highly abrasion resistant coating on the surface of the plastic substrate. Foremost among these are the coating compositions described in U.S. Pat. Nos. 3,986,997 and 4,027,073 to Clark, Nos. 4,177,175, 4,242,416, and 4,245,118 to Baney et al., No. 4,177,315 to Ubersax, Nos. 4,188,451 and 4,218,508 to Humphrey, Nos. 4,207,357, 4,242,381, and 4,242,383 to Goosens, No. 4,355,135 to January, Nos. 4,390,373 and 4,442,168 to White et al., Nos. 4,500,669, 4,540,634, 4,571,365 and 4,702,773 to Ashlock et al., No. 4,753,827 to Yoldas, and Nos. 5,013,608 and 5,102,695 to Guest et al. The hard coats described in these patents have been widely used to impart abrasion resistance to plastic lenses, including those used in eye glasses and safety goggles, windows in buildings and motor vehicles, plastic panels, and other articles where an abrasion resistant or scratch resistant surface coating is of importance. However, such abrasion resistant hard coats are also known to reduce the impact resistance of a plastic substrate in certain applications.
In recent years, plastic materials with increasingly higher refractive indexes have been used to make ophthalmic lenses. As their refractive index increases, the thickness of the lens required to achieve the same level of correction decreases. The resulting lens is thinner and lighter and, therefore, more attractive to the user. These higher refractive index plastic materials, however, still tend to be relatively soft and scratch more easily than glass. Known organic or siloxane coating compositions can be applied to these plastic materials to provide a protective abrasion resistant coating. However, when such a hard coat is applied to these higher refractive index plastic materials, their impact resistance tends to decrease in comparison to the uncoated plastic material. This is a serious disadvantage, particularly since the decrease may be large enough to reduce the impact resistance below minimum government standards. Moreover, application of additional coating layers, such as anti-reflective coatings, to these coated plastic materials may also reduce further their impact strength properties.
One approach to solving this problem is to apply an elastomeric primer layer of a thermoplastic polymer resin, e.g., a polyurethane, between the plastic substrate and the abrasion resistant coating. The elastomeric layer serves as an energy absorbing layer to prevent cracks that form in the hard coat upon impact from propagating into the plastic substrate. However, a thermoplastic resin lacks a crosslinked structure and can be adversely affected by solvent or monomer-containing overcoats. Consequently, when a plastic material to which a thermoplastic polymer primer layer has been applied is subsequently immersed in or coated with an organic or polysiloxane coating solution, the solvent or monomer therein may dissolve the thermoplastic primer layer and cause it to be eluted in the abrasion resistant coating solution. This has a deleterious affect on the transparency of the abrasion resistant coated article. The thermoplastic primer layer may also lose its transparency or become whitened as a result of being attacked by the solvent.
To avoid these problems, it has been suggested that the primer layer should consist of a thermoset polyurethane in at least one organic solvent, with the polyurethane being formed from a blocked isocyanate which requires the application of heat to disassociate the blocking agent so that the isocyanate group can react with the active hydrogen of the polyol and crosslink. Use of a blocked isocyanate prevents this reaction from taking place at room temperature. However, the addition of a heating step to cure the thermoset polyurethane primer layer before a protective hard coat can be applied over it is expensive, adds undue complexity to the process, and is generally not desirable. Moreover, the solvent(s) may also aggressively attack the plastic substrate.
Accordingly, the need exists for a process to improve the impact resistance of a plastic substrate which does not require the application of a thermoplastic polymer primer layer or heating a solvent-based thermoset polyurethane primer layer to cure it before another coating composition can be applied over it.